The present invention relates generally to superheterodyne receivers and, more particularly to an apparatus and method for processing Radio Frequency signal voltages, both pulse and amplitude modulated, of unknown frequency. Such processing is accomplished without the need for complex tuning mechanisms to generate a tracking local oscillator signal having a frequency proportional to the unknown signal frequency. The present invention obtains signal selectivity by means of a directional antenna whereas the prior art requires selectivity by means of manual frequency tuning of radio frequency state(s) and concurrent adjusting of the receiver's local oscillator. For certain applications such as radar search and for radio monitoring, manual tuning consumes more time than can be tolerated to isolate signal frequencies versus direction.
The prior art requires a receiver equipped with an antenna to be tuned in incremental stages to detect a desired signal. In the case wherein a directional antenna is used, tuning of the entire frequency band of the receiver must be accomplished for each angular increment of antenna position or vice-versa. The prior art of using radar with superheterodyne receivers for air-sea rescue and other beacon or transponder uses requires that each radar involved be tuned to the exact same frequency or that a separate local oscillator be used that is pre-set to a specific frequency that all transponders are set to. These requirements are overly stringent because of the requirements of stability for both the transponders and local oscillators that must be maintained at specific frequencies and the strong transponder reply necessary to overcome reflected clutter. In the present invention the local oscillator signal is generated automatically as a product of the unknown received signal and a fixed-pretuned stable oscillator whereas the prior art requires search tuning to generate the correct local oscillator signal that must track with the RF antenna tuning. The prior art for radar application samples the transmitted signal in setting the local oscillator frequency. As a result, for both radar and radio applications, the IF bandpass must be wide enough to accomodate various drifts in frequency and tracking as well as doppler changes in signal frequencies to significantly affect the gain-bandwidth product that establishes receiver sensitivity. This is not germane to the present invention as the required bandwidth is merely a function of pulsewidth and/or modulation side bands.